Polyamide resins have good mechanical properties, heat resistance and chemical resistance and are thereby favorably used in the applications of automobiles and electric and electronic parts. Polyamide resins have good heat aging resistance and are accordingly used for components exposed to extremely high-temperature heat such as engine covers, among various applications. With a recent increase in density of components in the automobile engine room and a recent increase in engine power, the environmental temperature in the engine room gradually increases. There is accordingly a need for the heat aging resistance in the higher temperature condition. A conventionally known technique to improve the heat aging resistance of the polyamide resin provides a polyamide resin composition by mixing a copper compound and a halogen compound with a polyamide resin (for example, JP 2006-273945 A). The polyamide resin composition of JP 2006-273945 A, however, fails to provide sufficient heat aging resistance responding to the recent increase in environmental temperature in use. Various technological improvements have been proposed as the technique to further improve the heat aging resistance at high temperature. For example, a polyamide resin composition has been proposed to include a polyamide resin, a polyol having the number-average molecular weight of less than 2000, an auxiliary stabilizer such as a copper stabilizer and hindered phenol, and a polymer reinforcing material (for example, US 2010/0029819 A). In another example, a polyamide resin composition has been proposed to include a polyamide resin, polyethyleneimine, a lubricant, a copper-containing stabilizer, a filler and nigrosin (for example, US 2011/0290209 A).
A molded product produced from the polyamide resin composition of US 2010/0029819 A has good heat aging resistance at a temperature of 150° C. to 230° C., but has a problem of poor heat aging resistance at lower than 150° C. The polyamide resin composition of US 2011/0290209 A also has good heat aging resistance at a temperature of 160° C. to 180° C., but has a problem of poor heat aging resistance at temperature lower than 150° C. Despite a yearly increasing tendency in environmental temperature of the components in the automobile engine room, the engine room is not always kept at high temperature. A material having the heat aging resistance not only at high temperature but at low temperature is needed for the components in the automobile engine room.
The polyamide resin compositions of US 2010/0029819 A and US 2011/0290209 A additionally have problems of (i) surface appearance such as bleed out of the polyol or coloring caused by the like on the surface layer of the molded product or isolation of the copper ion and (ii) poor retention stability. The automobile components such as engine room components having the demand for the heat aging resistance require the high dimensional accuracy, chemical resistance and creep resistance as well as the high heat aging resistance. Molded products produced from the polyamide resin compositions described in JP 2006-273945 A, US 2010/0029819 A and US 2011/0290209 A, however, fail to provide such properties at sufficient levels. By taking into account these problems, it could be helpful to provide a polyamide resin composition usable to produce a molded product having excellent heat aging resistance, surface appearance, retention stability, dimensional accuracy, chemical resistance and creep resistance.